Mr Trump did not elaborate on the Swedish reference at the time, leaving many Swedes baffled over the non-existent incident and their embassy in Washington "trying to get clarity".

But White House principal deputy press secretary Sarah Sanders said Mr Trump was referring to "rising crime and recent incidents in general, and not referring to a specific incident".

"I think he was referring to a report he had seen the previous night that spoke specifically to that topic," she said.

After she spoke, Mr Trump tweeted to confirm he was referring to a Fox News segment in which journalist Ami Horowitz, who had visited Sweden, claimed a link between rising crime and refugee admissions.

But official statistics show Sweden's crime rate has fallen since 2005, even as it has taken in hundreds of thousands of immigrants from war-torn countries like Syria and Iraq.

Sweden's embassy in the US repeated Mr Trump's tweet about having seen the Fox report, and added: "We look forward to informing the US administration about Swedish immigration and integration policies".

'What has he been smoking?'

Swedes were left scratching their heads and ridiculing Mr Trump's remark, prompting many to take to social media to discuss "last night in Sweden".

Swedish Foreign Ministry spokeswoman Catarina Axelsson said the Government was not aware of any "terror-linked major incidents".

Sweden's security police said it had no reason to change the terror threat level.

"Nothing has occurred which would cause us to raise that level," agency spokesman Karl Melin said.

Swedish Foreign Minister Margot Wallstrom appeared to respond to the statement by posting on Twitter an excerpt of a recent speech in which she said democracy and diplomacy "require us to respect science, facts and the media".